1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to optical burst switching networks, and more particularly, to suppressing a surge occurring at an optical amplifier in an optical burst switching network.
2. Description of the Related Art
In general, an optical signal may be classified into an optical circuit signal, an optical packet signal, or an optical burst signal. An optical circuit signal uses a preset optical path. In contrast, an optical packet signal and an optical burst signal do not use a preset optical path.
FIG. 1 illustrates nodes that transmit and receive an optical signal over an optical communication network. Hereinafter, operations of the nodes are described according to the type of the optical signal in reference to FIG. 1. It should be understood that each node includes an optical switch therein. The optical switch sets an optical path for the transmission of the optical signal according to a control signal. The transmission of an optical circuit signal is first explained.
FIG. 1 shows a node A 100 through a node E 108. It is assumed that node A 100 generates or receives an optical circuit signal destined for the node E 108. Thus, node A 100 is required to set an optical path to deliver the optical circuit signal to node E 108. To this end, node A 100 transmits to a node C 104 a route request message that contains the address of node A 100 (source address), an address of the node E 108 (destination address), and the type and the number of the optical signal. Node C 104 analyzes the destination address contained in the received route request message. Node C 104 then recognizes that the destination address of the route request message matches the address of node E 108. Thus, node C 104 establishes the optical path using the recognized location of node E 108.
Node C 104 transmits the received route request message to node E 108 along the optical path that is established. Node E 108 recognizes that the destination address of the received route request message is its address. Hence, node E 108 transmits a route response message along the optical path of the route request message. Node C 104 forwards the received route response message to node A 100.
Consequently, the optical path is established between node A 100 and node E 108. After the optical path is established, node A 100 transmits the optical circuit signal to node E 108. Note that node C 104 manipulates the optical switch to output the received optical circuit signal to node E 108. As mentioned above, the transmission of the optical circuit signal requires establishing the optical path in advance, which causes to delay the transmission time of the optical circuit signal. In addition, the established optical path for the transmission of the optical circuit signal is used exclusively until the transmission of the optical circuit signal is completed. In other words, the optical switch maintains the current switching state until the transmission of the optical circuit signal is completed.
The following is an explanation of the transmission of the optical packet signal. Upon generating the optical packet signal, node A 100 provides node C 104 with an optical label signal containing optical packet signal information. The information contained in the optical label signal is analogous to the information contained in the route request message used for the transmission of the optical circuit signal.
Node A 100 transmits the optical packet signal to node C 104 substantially concurrently with the optical label signal. Upon receiving the optical label signal, node C 104 can recognize that the following optical packet signal has to be delivered to node E 108. Hence, node C 104 manipulates the optical switch to output the received optical packet signal to node E 108. As a result, the signal transmission time can be reduced and the delivery path of the optical signal is not exclusively used, in contrast to transmission of an optical circuit signal.
The transmission scheme of the optical burst signal is similar to that of the optical packet signal. A difference lies in that the optical burst signal is transmitted by the burst while the optical packet signal is transmitted by the packet. The burst is an aggregation of a plurality of packets. Upon generating a plurality of optical packet signals, node A 100 generates a burst from the plurality of the optical packet signals and transmits the burst to node C 104.
Typically, the optical signal transmitted through an optical fiber decreases the power of the optical signal due to several reasons. In this regard, the optical fiber is provided with an optical amplifier to amplify the decreased power of the optical fiber at intervals. As illustrated in FIG. 1, an optical amplifier 110 is located between node C 104 and node E 108. Although FIG. 1 illustrates only one optical amplifier 110, more than one optical amplifier can be equipped in the optical communication network according to a user's setup. The following explains characteristics of optical amplifier 110.
Generally, optical amplifier 110 employs an erbium-doped fiber amplifier (EDFA). The EDFA uses 1.55 μm wavelength which has a minimal loss on the optical fiber. The EDFA incurs loss below 50 dB similar to a semiconductor laser amplifier, and the EDFA can carry out the amplification regardless of the polarization degree of the light.
FIG. 2 shows the relationship of a power and a gain of the input optical signal at the EDFA. The gain is the power of the output optical power with respect to the power of the input optical signal. Generally, the EDFA has the constant power of the output optical signal regardless of the power of the input optical signal. Accordingly, the EDFA operates to provide a high gain for an input optical signal having a low power and a low gain for an input optical signal having a large power. In FIG. 2, when the power of the input optical signal is −40 dBm, the gain is about 34 dB, and when the power of the input optical signal is 0 dBm, the gain is about 10 dB.
FIG. 3 depicts the powers of the input optical signal and the output optical signal at the EDFA. Particularly, FIG. 3 depicts a first section where the power of the input optical signal is zero, and a second section where the power of the input optical signal is X (X is an arbitrary constant more than zero). In the first section, as illustrated in FIG. 1, when the power of the input optical signal is zero, the power of the output optical signal is also zero regardless of the gain level. Meanwhile, even if the power of the input optical signal is zero, the EDFA operates to output the output optical signal with a higher gain, generally, the highest gain as shown in FIG. 2.
In the second section, the optical signal with the power X is input to the EDFA. The EDFA amplifies and outputs the input optical signal. Even when the power of the input optical signal changes at a specific point, the gain gradually changes over a certain time period. At the start point of the second section, the EDFA amplifies the power of the output optical signal to the gain corresponding to the power zero of the input optical signal, rather than to the gain corresponding to the power X of the input optical signal. Such amplification is illustrated in FIG. 3. In short, when the power of the input optical signal abruptly changes, the surge component is generated in the output optical signal.
The surge component shortens the life span of devices on the optical path, and causes damage to the devices. Furthermore, the surge component may result in distortion of the output optical signal. Therefore, a solution is required to suppress the surge component that occurs in the output optical signal.
FIG. 4 depicts a construction of a conventional node 420 which suppresses the surge component from the output optical signal. Node 420 includes an optical brancher 400, a delayer 402, an optical coupler 404, an optical detector 410, a controller 412, and an optical signal generator 414. The optical path, which carries the optical signal from node 420, includes an optical amplifier 406 and optical filter 408.
Optical brancher 400 branches the power of the optical signal input to node 420 into two. The wavelength of the optical signal fed into optical brancher 400 is λ1. Typically, optical brancher 400 branches the power of the input optical signal into the ratio of 9 to 1. Thus, optical brancher 400 provides delayer 402 with the branched optical signal having 9/10 power, and provides optical detector 410 with the optical signal having 1/10 power.
Delayer 402 delays the input optical signal over a certain time period and provides the delayed optical signal to optical coupler 404. The delay time at delayer 402 is a time taken for the optical signal fed to optical detector 410 to arrive at optical coupler 404 via controller 412 and optical signal generator 414. Optical detector 410 detects the power of the input, optical signal and provides the measured power and the input optical signal to controller 412.
Controller 412 determines whether the measured power of the optical signal equals zero. If the power of the optical signal is zero, controller 412 controls optical signal generator 414 to generate an optical signal having the same power as the power of an optical signal. If the measured power of the optical signal is not zero, controller 412 forwards the received optical signal to optical signal generator 414, rather than controlling generation of the optical signal.
Upon receiving the direction to generate the optical signal, optical signal generator 414 generates the optical power having a preset power and provides the generated optical signal to optical coupler 404. The wavelength of the optical signal generated at optical signal generator 414 is λd. If there is no direction to generate the optical signal, optical signal generator 414 forwards the received optical signal to optical coupler 404.
Optical coupler 404 couples the optical signals received from delayer 402 and optical signal generator 414. Next, optical coupler 404 outputs the optical signal having a constant power. Optical coupler 404 provides the coupled optical signal to optical amplifier 406. Optical amplifier 406 amplifies the optical signal fed from optical coupler 404, and optical filer 408 filters the input optical signal. Optical amplifier 406 amplifies the optical signal with a constant gain as the received optical signal having constant power. Optical filter 408 passes only the optical signal with the wavelength λ1 among the optical signals, and blocks the optical signal having the wavelength λd.
FIG. 5 depicts the optical signal output from the optical coupler 404. The optical signal is fed into optical coupler 400 is in a period from t0 to t1, and a period from t2 to t3. Optical signal generator 414 generates the optical signal in the period from t1 to t2, and the period after t3. In short, the optical signal generator 414 generates the optical signal during the periods in which the optical signal is not input to the optical brancher 400. In addition, the wavelength of the optical signal generated at optical signal generator 414 is similar to that of the optical signal input to optical brancher 400 so as to be blocked at optical filter 408, and that the power of the generated optical signal is the same as that of the optical power input to optical brancher 400.
However, when optical amplifier 406 suppresses the surge component as shown in FIG. 4, an additional construction is required to branch the optical signal and the input optical signal is subject to delay, as a result. Furthermore, as the optical signal passes through optical detector 410, controller 412, and optical signal generator 414, the optical signal is subject to distortion, loss, and the like.